It is common practice to mount various types of equipment, such as engines, machine tools, fans, blowers, pumps, compressors, turbines, indeed all manner of vibration-producing equipment, on vibration isolators for minimizing the transmission of vibration from the equipment to a supporting structure. On occasion, vibration isolation systems are used to minimize the transmission of vibration from the supporting structure to the equipment (a use often referred to as "negative isolation"); such is the case with delicate scientific equipment that is sensitive to ambient vibration generated by equipment in the building and vehicular traffic and other sources around the building.
In various installations of vibration isolated equipment, it is desirable to include restraint devices to prevent the equipment from moving relatively large distances in response to seismic events (earthquakes); if the natural frequency of the isolation system should be matched by an earthquake, possibly damaging high amplitude, transient vibrations of the equipment may occur. Moreover, the input force of the seismic event may be magnified substantially. Energy-absorbing stops may be adequate in many cases to reduce the excursion, but the stops always magnify the input forces.
The inventor of the present invention has previously disclosed an improved seismic restraint device in U.S. Pat. No. 4,040,590 (assigned to the assignee of the present invention), and that device is now in commercial use. That device employs friction elements to limit the maximum force transmitted to the isolated equipment to the input force of the earthquake. The device permits unrestrained, normal vibration of the equipment (normal operation of the isolators), while preventing magnification of seismic input forces. The friction elements limit the force transmitted to the equipment to an input force of a selected amount, and if the input force exceeds the restraining friction forces, the friction elements slip, and after a small movement, the equipment is stopped from further movement by resilient energy-absorbing stops, which absorb the remaining input energy.